This invention pertains to a process for thermal hydrodealkylation of alkylated phenols to produce increased yields of phenol product. It pertains particularly to such thermal hydrodealkylation process utilizing reaction temperatures below about 1100.degree. F. (593.degree. C.).
When a mixture of mono- and poly-alkylated phenols is subjected to hydrodealkylation reaction at elevated temperature and pressure conditions in the presence of hydrogen, but in the absence of a catalyst, to effect thermal hydrodealkylation reactions, not only are alkyl substituents removed but there is also a tendency for the hydroxyl group to be cleaved from the aromatic ring. Because phenol is usually the more desired product rather than benzene and alkylated benzenes, it is desirable to increase the selectivity of the thermal hydrodealkylation reaction to produce increased yields of phenol.
In the prior art, U.S. Pat. Nos. 3,284,513 and 3,284,514 to Dedinas et al., disclosed that selectivity to the alkyl substituents removal can be maximized and cleavage of hydroxyl group reduced by converting the mono- and poly-alkylated phenols to an extent of not more than about 88 to 92 V % conversion, when subjecting them to preferred reaction conditions of a temperature of about 900.degree. to 1600.degree. F. and a space velocity of about 0.2 to 3.5 volume feed/hour/volume reactor. The reaction temperature used in the examples was limited to the narrower range of 1106.degree.-1355.degree. F. No effect of temperature on the selectivity of dealkylation versus dehydroxylation was discovered. Also, U.S. Pat. No. 4,230,895 to Daly discloses a process for thermal dealkylation of alkylphenols at temperature of 1000.degree.-1500.degree. F. and total pressure of 400-800 psig in the presence of added water vapor. Although Daly discloses a reaction temperature range of 1000.degree.-1500.degree. F., his experiments were confined to 1150.degree. and 1200.degree. F. with added water vapor; therefore, he did not recognize any yield advantage of using reaction temperatures below about 1100.degree. F. without added water. Thus, a need remains for further process improvements for selectively producing phenol from alkylated phenols, whereby the alkyl substituent in the alkylated phenol may be removed without simultaneously removing the hydroxyl group.